As is known, a gas mask can be constituted by several parts, the primary parts being a filter and a fitting to connect it to the mask. The fitting essentially comprises a nozzle for screwing the filter and, behind the nozzle, a chamber into which is conveyed the already filtered air.
A check valve is positioned between the nozzle and the chamber; as is known, the check valve includes a diaphragm of elastomeric material, whose flexion, originated by a depression, allows air to pass from the filter to the chamber situated behind the nozzle.
It has been noted that in many cases the user of the mask must face the unforeseen event that the filter becomes exhausted, this being due to, for instance, an activity being carried out for a time longer than it was considered necessary beforehand.
In these circumstances, the user, remaining in condition of apnea and having available a new filter, might effect the various steps needed to replace the filter, namely: to remove the exhausted filter from the mask nozzle, to take a new filter from a pocket or from an appropriate container, and to screw it on the mask nozzle.
As can be readily understood, the above-indicated steps, to be carried out according to an apparently regular sequence, might be influenced, in practice, by the psychological condition of the operator, which is, of course, different for the various users, with consequent differentiated risks which might lead to a temporary interruption of the apnea, with partial inhalation of air and therefore of noxious substances highly harmful to the user's health.
Also, it is to be considered that the need of replacing an exhausted filter with a new one has a considerable importance in the event of a wide contamination in open air as, for instance, in proximity of chemical or nuclear plants in consequence of serious accidents, namely, in events where the time to reach the area of intervention from non-contaminated zones and then to return to the base is approximately the same, and sometimes longer, than the average life of a filter.
The possibility of providing a mask with a device which enables the user to remain a long time in a contaminated atmosphere, by interrupting the inflow of polluted air during the replacement of the filter, is therefore highly desirable in present masks.
Unfortunately, heretofore a satisfactory solution to this problem has not been found, since the problems to be solved are several and, inter alia, contrast with one another. In fact, one problem is that of providing and installing an additional element or device to be incorporated with the usual accessories for gas masks without however modifying the mask structure and shape already established in accordance with legal provisions currently in force.
An optimum solution, moreover, must intercept the air flow paths from the outside to the inside of the mask during the substitution of the filter without affecting the regular operation of the air flow paths when the filter is inserted in the mask nozzle.
Another problem is presented by the necessity of adding further parts to those already existing in a mask without giving rise, due to the additional weight, to conditions of intolerance or weariness for the user.